Polypropylene resins are used in daily use sundries, kitchen utensils, packing films, household electrical appliances, mechanical parts, electrical components, automobile parts and other various fields. The polypropylene resins are incorporated with various additives in conformity with the demanded performance. In particular, in the use in automobile parts, compositions comprising a polypropylene resin incorporated with an α-olefin copolymer rubber and an inorganic filler such as talc are utilized in large quantities because molded articles exhibit an excellent balance of rigidity and impact resistance.
Automobile parts for inner and outer trims are often formed by injection molding in view of productivity. When an injection molding of the above polypropylene resin composition is carried out, a plurality of periodic striped patterns known as flow marks or tiger marks occur on the surface of injection molded articles in directions crossing with the direction of injection flow, which periodic striped patterns are conspicuous. When flow marks having occurred on the surface of a molded article are conspicuous, the appearance of the molded article would be degraded. Thus, according to necessity, coating or other measures would be carried out in order to cause the flow marks to disappear. For eliminating the flow marks of the molded article obtained from the above polypropylene resin composition or rendering the flow marks inconspicuous, various methods are employed. For example, a method in which use is made of a metal mold whose temperature can be varied and a resin is injected into the metal mold maintained at a high temperature is employed. However, this method has a problem in production in that a special metal mold is needed and in that the molding cycle is prolonged.
Moreover, with respect to automobile parts for inner trims, the surface thereof is furnished with a pattern known as an emboss for the purpose of decorating the surface or lowering the surface gloss. This emboss is formed by transferring a pattern provided on the inside surface of the metal mold for injection molding to the surface of molded article at the time of injection molding. When an injection molding of the above polypropylene resin composition is conducted, there would occur such problems that the transfer of emboss pattern provided on the inside surface of the metal mold is poor to thereby fail to lower the gloss, and that the transferability is different between the vicinity of a gate of the metal mold and a position remote from the gate to thereby bring about a gloss difference.
In the injection molding, pellets of solid resin composition are heated and melted (plasticized) in a heated cylinder, and the molten resin composition is injected and charged into a metal mold (cavity). The heating and melting of the resin composition used to obtain an injection molded article in the subsequent molding cycle are carried out during the cooling after the completion of the injection and charging. For shortening the molding cycle for obtaining an injection molded article, it is necessary to shorten the cooling time. There would be no problem if the heating and melting of resin composition needed in the subsequent injection and charging can be completed during the shortened cooling time. However, when the heating and melting cannot be completed during the cooling time, the subsequent injection and charging cannot be effected. This causes shortening of the cooling time impracticable. The above polypropylene resin composition necessitates a prolonged heating/melting time (plasticization time) and disenables shortening of the cooling time. Therefore, the above polypropylene resin composition has a drawback in that the molding cycle cannot be shortened.
A composition comprising a polypropylene resin (propylene homopolymer, propylene block copolymer) produced in the presence of a metallocene catalyst, an α-olefin copolymer rubber and an inorganic filler is known as one described in Japanese Patent Laid-open Publication No. 10(1998)-1573. However, in the production of the polypropylene resin in the presence of a metallocene catalyst, 1,3-insertion or 2,1-insertion of propylene occurs in a proportion of about 1%. Thus, the crystallinity of the polypropylene resin is low, so that the melting point thereof is about 150° C., which melting point is lower than 160° C. of the polypropylene resin produced in the presence of a titanium catalyst. Further, the polypropylene resin produced in the presence of a metallocene catalyst is inferior to the polypropylene resin produced in the presence of a titanium catalyst in the tensile strength properties, flexural strength properties, rigidity, etc. Accordingly, the mechanical strength properties of the composition comprising the polypropylene resin produced in the presence of a metallocene catalyst, an α-olefin copolymer rubber and an inorganic filler are poor as compared with those of the composition comprising the polypropylene resin produced in the presence of a titanium catalyst, an α-olefin copolymer rubber and an inorganic filler. Therefore, the former composition has not been put to practical use.
The inventors have made extensive and intensive studies with a view toward solving the above problems. As a result, it has been found that there is substantially no 1,3-insertion or 2,1-insertion of propylene in the propylene homopolymer or propylene block copolymer produced in the presence of a specified metallocene catalyst, so that the melting point of the propylene homopolymer or propylene block copolymer is high and so that the molded article thereof is excellent in the rigidity, tensile strength properties and flexural strength properties. It has further been found that the polypropylene resin composition based on the propylene homopolymer or propylene block copolymer exhibits excellent performance as a composition for automobile part. The present invention has been completed on the basis of these findings.
It is an object of the present invention to solve the above problems of the prior art, specifically to provide an automobile part, especially an injection molded automobile part, which is improved in flow marks and has excellent appearance and emboss transfer.
It is another object of the present invention to provide an automobile part, especially an injection molded automobile part, which has an excellent balance of rigidity and impact resistance.